In recent years, metal halide lamps in which a translucent ceramic is used instead of translucent quartz as an arc tube member have been widely used. Translucent ceramic materials such as a translucent alumina ceramic has the advantage of excellent corrosion resistivity at high temperature against a metal halide which is a filler of the metal halide lamp, compared to conventional translucent quartz materials. Therefore, when a ceramic is used as an arc tube member, it is possible to improve luminous efficacy and color rendering of the lamp by setting arc tube temperature high during operation.
However, ceramic materials such as an alumina ceramic have a drawback that they are more fragile to thermal shock than quartz materials. This is because the coefficient of thermal expansion of ceramics is larger than that of quartz. For example, the coefficient of thermal expansion of quartz glass is about 0.5×10−6/° C., while the coefficient of thermal expansion of alumina ceramics is about 8×10−6/° C. in the temperature range of 0 to 900° C. Thus, the coefficient of thermal expansion of alumina ceramics is about one digit larger than that of quartz.
As such a metal halide lamp using translucent ceramics such as alumina ceramics in an arc tube (hereinafter, referred to as ceramic metal halide lamp), those having a rated lamp wattage of not more than 400 W have been brought into practice. The term “rated lamp wattage” used herein represents typical power consumption of lamps declared in catalogue or the like.
However, a metal halide lamp having a rated lamp wattage of greater than or equal to 450 W has not been brought into practice. This is because of the aforementioned characteristic of a ceramic material, namely, being more fragile to thermal shock than quartz materials. Accordingly, in attempting implementation of a ceramic metal halide lamp having a rated lamp wattage of greater than or equal to 450 W, a problem arises that the ceramic arc tube cracks due to rapid increase in arc tube temperature during operation of the lamp.
Japanese Unexamined Patent Publication No. 2003-086130 proposes one measure for solving the above problem of cracking of an arc tube in a ceramic metal halide lamp having large lamp power. FIG. 5 is a section view of an arc tube of the ceramic metal halide lamp disclosed in the above publication. In FIG. 5, the numeral 21 denotes an electrode, 22 denotes an electricity introducing member, 23 denotes an arc tube (translucent ceramic tube), 24 denotes a narrow tube, 27 denotes a second coil, and 28 denotes a sealing material.
In this publication, the arc tube 23 made of a translucent ceramic in which cerium iodide and potassium iodide are enclosed as luminescent substances is provided; the molar composition ratio of the luminescent substances NaI/CeI3 is set within the range of 3.8 to 10; and Le/D is defined in the ranges of 0.75 to 1.70, 0.80 to 1.80, 0.85 to 1.90, 1.00 to 2.00 and 1.15 to 2.10 at the lamp watt of 200 W, 300 W, 400 W, 700 W and 1000 W, respectively, when assuming an electrode-to-electrode distance as Le, and a tube inner diameter of the arc tube as D within the range of a bulb wall loading we of the arc tube of 13 to 23 W/cm2, whereby the arc tube is prevented from cracking.
[Patent document 1] Japanese Unexamined Patent Publication No. 2003-86130